The identification and characterization of a family of prolactin-related proteins expressed according to a temporally specific and cell specific pattern in the placenta of several species has naturally led to the question of the functions of these proteins. Studies are proposed that will address what are the roles of rat placental members of the family - rat prolactin-like protein A (rPLP-A), rat prolactin-like protein B (rPLP-B), rat placental lactogen I (rPLI) and rat placental lactogen I-related protein. Radioreceptor assays will be established to determine the target tissues for these proteins. In the instances where the proteins bind to the lactogenic receptor the relative affinities of these proteins will be compared to those of prolactin and hGH. To determine where these proteins circulate - maternal serum, fetal serum, amniotic fluid - and at what levels, radioimmunoassays will be established. Proteins purified from placental explant medium using specific antibody columns will be used initially in these studies. Larger quantities of proteins will be required for future functional studies such as potential effects on the maintenance of progesterone secretion by the corpus luteum, the development of the mammary gland or on regulating prolactin secretion. Protein will be isolated and purified from CHO cells transfected with cDNA clones for the various proteins. These proteins will also be purified by immunoaffinity and tested in the RRA and RIA to determine that they are functionally equivalent to the native placental proteins. The functional studies to be carried out will be determined on the basis of the identification of target tissues and the location of circulating proteins. The newly identified rPLI-related protein of late pregnancy will be more completely characterized, in particular with relation to its ability bind to lactogenic receptors and stimulate Nb2 cells. The number of rPLI genes will be investigated.